1. Field of the Invention
The invention relates to a capstan for use in a magnetic tape transport system, and more particularly to a capstan which has a low mass and a very low moment of inertia to diameter ratio.
2. Description of the Prior Art
There are many capstans presently in use in magnetic tape transport systems for accelerating a length of magnetic tape. U.S. Pat. No. 3,261,563 entitled Magnetic Tape Reel Control Servo System, issued to Jesse I. Aweida, Donald K. Close and Henry C. Pao on July 19, 1966, teaches a magnetic tape transport system which accelerates a length of magnetic tape to 200 inches per second (ips) in 0.065 inches of magnetic tape. There are three variables, moment of inertia (I), torque (T), and radius of the capstan (R.sub.c), in the equation for determining linear acceleration of a length of magnetic tape. The equation determining linear acceleration (a) is: EQU a = [(T/I) .times. 2 R.sub.c ].
The larger the radius of the capstan (R.sub.c) is, the higher its linear acceleration (a) will be for a given torque (T) and a given moment of inertia (I). Similarly, the lower its moment of inertia (I) is, the higher its linear acceleration (a) will be for a given torque (T) and a given radius of the capstan (R.sub.c). The advantage of a capstan having a relatively large radius with a lower moment of inertia is that a capstan motor which drives the capstan may achieve a higher acceleration performance with the same power input, or conversely, the capstan motor may achieve the same acceleration performance with a lower power input. The capstans presently in use that have low moments of inertia have small radii; conversely, the capstans presently in use that have larger radii than the capstans having low moments of inertia have high moments of inertia.
U.S. Pat. No. 3,122,295, entitled Web Transport, issued to Richard H. Davison, John G. Simon and James O. Esselstyn on Feb. 25, 1964, teaches a capstan with a grooved surface for generating a proper air bearing between the capstan and a length of magnetic tape. U.S. Pat. No. 3,143,267, entitled Tape Handling Device, issued to Alexander R. Maxey on Aug. 4, 1964, teaches a capstan constructed as a hollow cylindrical pressure housing, the cylindrical walls of which are formed of a porous material and the interior cavity of which is coupled to through a hollow capstan drive shaft. This capstan also has a number of circumferential grooves formed in parallel planes normal to the axis of the capstan.
Most capstans are made of a lightweight metal such as aluminum in order to provide the necessary strength. However since almost all metals including aluminum have a relatively high elastic modulus, a metal capstan may be required to have considerable mass to maintain the required structural integrity while shipping, assembling and otherwise handling the capstan. If the capstan is too thin or is overloaded so as to exceed its elastic limit, the capstan will become bent or otherwise permanently distorted and thereby rendered useless. In effort to achieve low inertia by minimizing the amount of material used for construction, the strength of a conventional capstan is often reduced to such an extent that the mere inadvertent dropping of such a capstan on a hard surface if often enough to damage the capstan beyond repair in view of the close tolerances involved.
U.S. Pat. No. 3,930,603, entitled Low Inertia Capstan, issued to Frederic F. Grant on Jan. 6, 1976, teaches a capstan formed principally of thin plastic so as to decrease its mass and inertia several orders of magnitude with respect to metal capstans of comparable size without increasing its susceptibility to damage. At the same time however such capstan are constructed so as to possess the necessary strength and rigidity in a circumferential direction in which accelerating torques are applied. The inherent elasticity and thinness of the plastic parts of the capstan enable the capstan to undergo substantial resilient deformation in other than circumferential and radial directions exceeding its elastic limit while the design shape provides substantial resistance to significant deformation in the circumferential and radial directions as a result of tape forces. The capstan includes a pair of extremely thin resiliently deformable plastic webs of partially conical configuration. The webs are mounted on the opposite ends of a generally cylindrical inner hub which is substantially of plastic construction and which is rotatable about a central axis thereof. The webs are so arranged as to be separated from one another by a maximum distance at the inner hub and to draw closer together in directions toward their outer edges which support a very thin, hollow, generally cylindrical outer rim of plastic construction. The mass and resulting inertia may be even further minimized by cutting out and removing portions of the webs to define a plurality of spokes. The outer rim may be formed by turning on a lathe and finish grinding or alternatively by vacuum deep drawing.
The moment of inertia of an object is related to the mass (m) of the object and the distribution of the mass about an axis. Generally, the moment of inertia may be approximated by the equation: EQU I = [k .times. (m) .times. R.sub.o.sup.2 ]
where k is generally a coefficient varying between 1/2 and 1 and R.sub.o is the radius of the object. It is therefore a matter of logic that in order to reduce the moment of inertia of the object one must reduce either its mass or its radius. Reducing the radius of the object results in lowering the moment of inertia, but the lower moment of inertia is partially offset by the smaller radius which results in a higher angular acceleration to achieve the same linear acceleration. Reducing the mass of the object also lowers the moment of inertia, but the object loses structural strength as a result of lowering its mass.
Each of the prior art capstans includes a pair of discs, each of which is formed from a substantially thin, metal or plastic material and which is of a particular diameter, and a cylindrical tape band, which is of a diameter the same as the diameter of the discs. The tape band has a number of air holes distributed throughout its face so that a vacuum supply may pull the magnetic tape against the tape band and create friction between the tape band and the magnetic tape. The necessity of these air holes requires that additional mass be added to reinforce the capstan near its tape band because the air holes weaken the structural strength of the tape band. Further compounding this structural reinforcement problem, requiring additional mass, is that the discs must also be airtight in order for the vacuum supply to operate. This additional requirement means that the discs may not have any mass, unnecessary for adding to the structural strength of the capstan, removed.
In operation these vacuum holes cause air to flow unevenly over the tape band creating a non-uniform gap between the magnetic tape and the tape band. The non-uniform gap slows the acceleration of the capstan and also produces an uneven acceleration. In order to fabricate this type of capstan the two discs are bonded together to the tape band to form an air-tight assembly. This fabrication process is relatively expensive.
Another type of prior art capstan uses an adhesive coating on its cylindrical tape band to frictionally drive the magnetic tape, but there is no way to reduce this friction to zero as there is with a reversible air flow supply. All these surfaces of both types of the tape bands are presently machined to high tolerances by precision tooling in order to minimize speed variations. Furthermore, not only is the structural strength of the tape band critical to the durability of the capstans, but also the distribution of its mass is critical to the performance of the capstans in magnetic tape transport systems.
As a result of aerospace technology there are processes wherein a platen, which is formed from a material that is dissolvable in a selected solution, is plated with a material that is not dissolvable in selected solution. Some of the more common material forming the platen includes aluminum, soap, sodium bicarbonate, the non-active ingredients of pills which dissolve in water and which may be temporarily protected by a protective coating, or any other material which can be dissolved or etched away by a selected solution. Some of the more common plating materials include titanium, nickel, magnesium, aluminum and various plastic materials. However, the designs of the prior art capstans make this plating and dissolving or etching process unfeasible because there are manufacturing problems inherent in the vacuum hole design.